1. Field of the Invention
This invention relates to active filters, and more particularly to the compensation of active filters utilizing switched resistances for frequency variation in order to maintain relatively constant the Q and voltage gain of the circuit with variations in operating frequency.
2. Prior Art
The use of active filters in order to provide precision tuning of operating circuits is known in the prior art. In such filters tuning is typically accomplished by the control of voltage-variable impedances, by duty cycle switching of resistances, or by switching of parallel or series connections of resistance combinations.
Tuning by control of voltage-variable impedances, however, is susceptible to errors caused by extreme variations in tuning elements between units, as well as by variations in element characteristics with temperature and other environmental parameters. Tuning by variation of the repetition rate of filter elements inserted and withdrawn from the circuit is impractical for operation at high frequency.
For parallel resistance switching, large errors in Q and voltage gain occur for higher frequency filter designs, for which capacitance must be reduced.